To perform unplanned maintenance in the event of a
failure

The
machines needing unplanned maintenance do not include
mirror nodes. Follow the single machine procedure for each failed machine,
one DRU at a time. HADB service is not interrupted.

If the database state is Non-Operational:

The machines
needing unplanned maintenance include mirror nodes. One such case is when
the entire HADB is on a single failed machine. Get all the machines up and
running first. Then clear HADB and recreate the session store. See Clearing a database. This interrupts HADB service.